Network Working Group K. Konishi
Request for Comments: 3743 K. Huang
Category: Informational H. Qian
Y. Ko
April 2004
Joint Engineering Team (JET) Guidelines forInternationalized Domain Names (IDN) Registration andAdministration for Chinese, Japanese, and Korean
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
IESG Note
The IESG congratulates the Joint Engineering Team (JET) on developing
mechanisms to enforce their desired policy. The Language Variant
Table mechanisms described here allow JET to enforce language-based
character variant preferences, and they set an example for those who
might want to use variant tables for their own policy enforcement.
The IESG encourages those following this example to take JET's
diligence as an example, as well as its technical work. To follow
their example, registration authorities may need to articulate
policy, develop appropriate procedures and mechanisms for
enforcement, and document the relationship between the two. JET's
LVT mechanism should be adaptable to different policies, and can be
considered during that development process.
The IETF does not, of course, dictate policy or require the use of
any particular mechanisms for the implementation of these policies,
as these are matters of sovereignty and contract.
Abstract
Achieving internationalized access to domain names raises many
complex issues. These are associated not only with basic protocol
design, such as how names are represented on the network, compared,
and converted to appropriate forms, but also with issues and options
for deployment, transition, registration, and administration.
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RFC 3743 JET Guidelines for IDN April 20041. Introduction
Domain names form the fundamental naming architecture of the
Internet. Countless Internet protocols and applications rely on
them, not just for stability and continuity, but also to avoid
ambiguity. They were designed to be identifiers without any language
context. However, as domain names have become visible to end users
through Web URLs and e-mail addresses, the strings in domain-name
labels are being increasingly interpreted as names, words, or
phrases. It is likely that users will do the same with languages of
differing character sets, such as Chinese, Japanese and Korean (CJK),
in which many words or concepts are represented using short sequences
of characters.
The introduction of what are called Internationalized Domain Names
(IDN) amplifies both the difficulty of putting names into identifiers
and the confusion that exists between scripts and languages.
Character symbols that appear (or actually are) identical, or that
have similar or identical semantics but that are assigned the
different code points, further increase the potential for confusion.
DNS internationalization also affects a number of Internet protocols
and applications and creates additional layers of complexity in terms
of technical administration and services. Given the added
complications of using a much broader range of characters than the
original small ASCII subset, precautions are necessary in the
deployment of IDNs in order to minimize confusion and fraud.
The IETF IDN Working Group [IDN-WG] addressed the problem of handling
the encoding and decoding of Unicode strings into and out of Domain
Name System (DNS) labels with the goal that its solution would not
put the operational DNS at any risk. Its work resulted in one
primary protocol and three supporting ones, respectively:
1. Internationalizing Host Names in Applications [IDNA]
2. Preparation of Internationalized Strings [STRINGPREP]
3. A Stringprep Profile for Internationalized Domain Names
[NAMEPREP]
4. Punycode [PUNYCODE]
IDNA, which calls on the others, normalizes and transforms strings
that are intended to be used as IDNs. In combination, the four
provide the minimum functions required for internationalization, such
as performing case mappings, eliminating character differences that
would cause severe problems, and specifying matching (equality).
They also convert between the resulting Unicode code points and an
ASCII-based form that is more suitable for storing in actual DNS
labels. In this way, the IDNA transformations improve a user's
chances of getting to the correct IDN.
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Addressing the issues around differing character sets, a primary
consideration and administrative challenge involves region-specific
definitions, interpretations, and the semantics of strings to be used
in IDNs. A Unicode string may have a specific meaning as a name,
word, or phrase in a particular language but that meaning could vary
depending on the country, region, culture, or other context in which
the string is used. It might also have different interpretations in
different languages that share some or all of the same characters.
Therefore, individual zones and zone administrators may find it
necessary to impose restrictions and procedures to reduce the
likelihood of confusion, and instabilities of reference, within their
own environments.
Over the centuries, the evolution of CJK characters, and the
differences in their use in different languages and even in different
regions where the same language is spoken, has given rise to the idea
of "variants", wherein one conceptual character can be identified
with several different Code Points in character sets for computer
use. This document provides a framework for handling such variants
while minimizing the possibility of serious user confusion in the
obtaining or using of domain names. However, the concept of variants
is complex and may require many different layers of solutions. This
guideline offers only one of those solution components. It is not
sufficient by itself to solve the whole problem, even with zone-
specific tables as described below.
Additionally, because of local language or writing-system
differences, it is impossible to create universally accepted
definitions for which potential variants are the same and which are
not the same. It is even more difficult to define a technical
algorithm to generate variants that are linguistically accurate.
That is, that the variant forms produced make as much sense in the
language as the originally specified forms. It is also possible that
variants generated may have no meaning in the associated language or
languages. The intention is not to generate meaningful "words" but
to generate similar variants to be reserved. So even though the
method described in this document may not always be linguistically
accurate, nor does it need to be, it increases the chances of getting
the right variants while accepting the inherent limitations of the
DNS and the complexities of human language.
This document outlines a model for such conventions for zones in
which labels that contain CJK characters are to be registered and a
system for implementing that model. It provides a mechanism that
allows each zone to define its own local rules for permitted
characters and sequences and the handling of IDNs and their variants.
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RFC 3743 JET Guidelines for IDN April 2004
The document is an effort of the Joint Engineering Team (JET), a
group composed of members of CNNIC, TWNIC, KRNIC, and JPNIC as well
as other individual experts. It offers guidelines for zone
administrators, including but not limited to registry operators and
registrars and information for all domain names holders on the
administration of domain names that contain characters drawn from
Chinese, Japanese, and Korean scripts. Other language groups are
encouraged to develop their own guidelines as needed, based on these
guidelines if that is helpful.
2. Definitions, Context, and Notation2.1. Definitions and Context
This document uses a number of special terms. In this section,
definitions and explanations are grouped topically. Some readers may
prefer to skip over this material, returning, perhaps via the index
to terminology in section 7, when needed.
2.1.1. IDN
IDN: The term "IDN" has a number of different uses: (a) as an
abbreviation for "Internationalized Domain Name"; (b) as a fully
qualified domain name that contains at least one label that contains
characters not appearing in ASCII, specifically not in the subset of
ASCII recommended for domain names (the so-called "hostname" or "LDH"
subset, see RFC1035 [STD13]); (c) as a label of a domain name that
contains at least one character beyond ASCII; (d) as a Unicode string
to be processed by Nameprep; (e) as a string that is an output from
Nameprep; (f) as a string that is the result of processing through
both Nameprep and conversion into Punycode; (g) as the abbreviation
of an IDN (more properly, IDL) Package, in the terminology of this
document; (h) as the abbreviation of the IETF IDN Working Group; (g)
as the abbreviation of the ICANN IDN Committee; and (h) as standing
for other IDN activities in other companies/organizations.
Because of the potential confusion, this document uses the term "IDN"
as an abbreviation for Internationalized Domain Name and,
specifically, in the second sense described in (b) above. It uses
"IDL," defined immediately below, to refer to Internationalized
Domain Labels.
2.1.2. IDL
IDL: This document provides a guideline to be applied on a per-zone
basis, one label at a time. Therefore, the term "Internationalized
Domain Label" or "IDL" will be used instead of the more general term
"IDN" or its equivalents. The processing specifications of this
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RFC 3743 JET Guidelines for IDN April 2004
document may be applied, in some zones, to ASCII characters also, if
those characters are specified as valid in a Language Variant Table
(see below). Hence, in some zones, an IDL may contain or consist
entirely of "LDH" characters.
2.1.3. FQDN
FQDN: A fully qualified domain name, one that explicitly contains all
labels, including a Top-Level Domain (TLD) name. In this context, a
TLD name is one whose label appears in a nameserver record in the
root zone. The term "Domain Name Label" refers to any label of a
FQDN.
2.1.4. Registrations
Registration: In this document, the term "registration" refers to the
process by which a potential domain name holder requests that a label
be placed in the DNS either as an individual name within a domain or
as a subdomain delegation from another domain name holder. In the
case of a successful registration, the label or delegation records
are placed in the relevant zone file, or, more specifically, they are
"activated" or made "active" and additional IDLs may be reserved as
part of an "IDL Package" (see below). The guidelines presented here
are recommended for all zones, at any hierarchy level, in which CJK
characters are to appear and not just domains at the first or second
level.
2.1.5. RFC3066RFC3066: A system, widely used in the Internet, for coding and
representing names of languages [RFC3066]. It is based on an
International Organization for Standardization (ISO) standard for
coding language names [ISO639], but expands it to provide additional
precision.
2.1.6. ISO/IEC 10646
ISO/IEC 10646: The international standard universal multiple-octet
coded character set ("UCS") [IS10646]. The Code Point definitions of
this standard are identical to those of corresponding versions of the
Unicode standard (see below). Consequently, the characters and their
coding are often referred to as "Unicode characters."
2.1.7. Unicode Character
Unicode Character: The term "Unicode character" is used here in
reference to characters chosen from the Unicode Standard Version 3.2
[UNICODE] (and hence from ISO/IEC 10646). In this document, the
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characters are identified by their positions, or "Code Points." The
notation U+12AB, for example, indicates the character at the position
12AB (hexadecimal) in the Unicode 3.2 table. For characters in
positions above FFFF, i.e., requiring more than sixteen bits to
represent, a five to eight-character string is used, such as U+112AB
for the character in position 12AB of plane 1.
2.1.8. Unicode String
Unicode String: "Unicode string" refers to a string of Unicode
characters. The Unicode string is identified by the sequence of the
Unicode characters regardless of the encoding scheme.
2.1.9. CJK Characters
CJK Characters: CJK characters are characters commonly used in the
Chinese, Japanese, or Korean languages, including but not limited to
those defined in the Unicode Standard as ASCII (U+0020 to U+007F),
Han ideographs (U+3400 to U+9FAF and U+20000 to U+2A6DF), Bopomofo
(U+3100 to U+312F and U+31A0 to U+31BF), Kana (U+3040 to U+30FF),
Jamo (U+1100 to 11FF and U+3130 to U+318F), Hangul (U+AC00 to U+D7AF
and U+3130 to U+318F), and the respective compatibility forms. The
particular characters that are permitted in a given zone are
specified in the Language Variant Table(s) for that zone.
2.1.10. Label String
Label String: A generic term referring to a string of characters that
is a candidate for registration in the DNS or such a string, once
registered. A label string may or may not be valid according to the
rules of this specification and may even be invalid for IDNA use.
The term "label", by itself, refers to a string that has been
validated and may be formatted to appear in a DNS zone file.
2.1.11. Language Variant Table
Language Variant Table: The key mechanisms of this specification
utilize a three-column table, called a Language Variant Table, for
each language permitted to be registered in the zone. Those columns
are known, respectively, as "Valid Code Point", "Preferred Variant",
and "Character Variant", which are defined separately below. The
Language Variant Tables are critical to the success of the guideline
described in this document. However, the principles to be used to
generate the tables are not within the scope of this document and
should be worked out by each registry separately (perhaps by adopting
or adapting the work of some other registry). In this document,
"Table" and "Variant Table" are used as short forms for Language
Variant Table.
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RFC 3743 JET Guidelines for IDN April 20042.1.12. Valid Code Point
Valid Code Point: In a Language Variant Table, the list of Code
Points that is permitted for that language. Any other Code Points,
or any string containing them, will be rejected by this
specification. The Valid Code Point list appears as the first column
of the Language Variant Table.
2.1.13. Preferred Variant
Preferred Variant: In a Language Variant Table, a list of Code Points
corresponding to each Valid Code Point and providing possible
substitutions for it. These substitutions are "preferred" in the
sense that the variant labels generated using them are normally
registered in the zone file, or "activated." The Preferred Code
Points appear in column 2 of the Language Variant Table. "Preferred
Code Point" is used interchangeably with this term.
2.1.14. Character Variant
Character Variant: In a Language Variant Table, a second list of Code
Points corresponding to each Valid Code Point and providing possible
substitutions for it. Unlike the Preferred Variants, substitutions
based on Character Variants are normally reserved but not actually
registered (or "activated"). Character Variants appear in column 3
of the Language Variant Table. The term "Code Point Variants" is
used interchangeably with this term.
2.1.15. Preferred Variant Label
Preferred Variant Label: A label generated by use of Preferred
Variants (or Preferred Code Points).
2.1.16. Character Variant Label
Character Variant Label: A label generated by use of Character
Variants.
2.1.17. Zone Variant
Zone Variant: A Preferred or Character Variant Label that is actually
to be entered (registered) into the DNS. That is, into the zone file
for the relevant zone. Zone Variants are also referred to as Zone
Variant Labels or Active (or Activated) Labels.
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RFC 3743 JET Guidelines for IDN April 20042.1.18. IDL Package
IDL Package: A collection of IDLs as determined by these Guidelines.
All labels in the package are "reserved", meaning they cannot be
registered by anyone other than the holder of the Package. These
reserved IDLs may be "activated", meaning they are actually entered
into a zone file as a "Zone Variant". The IDL Package also contains
identification of the language(s) associated with the registration
process. The IDL and its variant labels form a single, atomic unit.
2.2. Notation for Ideographs and Other Non-ASCII CJK Characters.
For purposes of clarity, particularly in regard to examples, Han
ideographs appear in several places in this document. However, they
do not appear in the ASCII version of this document. For the
convenience of readers of the ASCII version, and some readers not
familiar with recognizing and distinguishing Chinese characters, most
uses of these characters will be associated with both their Unicode
Code Points and an "asterisk tag" with its corresponding Chinese
Romanization [ISO7098], with the tone mark represented by a number
from 1 to 4. Those tags have no meaning outside this document; they
are a quick visual and reading reference to help facilitate the
combinations and transformations of characters in the guideline and
table excerpts.
3. Scope of the Administrative Guidelines
Zone administrators are responsible for the administration of the
domain name labels under their control. A zone administrator might
be responsible for a large zone, such as a top-level domain (TLD),
whether generic or country code, or a smaller one, such as a typical
second- or third-level domain. A large zone is often more complex
than its smaller counterpart. However, actual technical
administrative tasks, such as addition, deletion, delegation, and
transfer of zones between domain name holders, are similar for all
zones.
This document provides guidelines for the ways CJK characters should
be handled within a zone, for how language issues should be
considered and incorporated, and for how Domain Name Labels
containing CJK characters should be administered (including
registration, deletion, and transfer of labels).
Other IDN policies, such as the creation of new top-level domains
(TLDs), the cost structure for registrations, and how the processes
described here get allocated between registrar and registry if the
zone makes that distinction, also are outside the scope of this
document.
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Technical implementation issues are not discussed here either. For
example, deciding which guidelines should be implemented as registry
actions and which should be registrar actions is left to zone
administrators, with the possibility that it will differ from zone to
zone.
3.1. Principles Underlying These Guidelines
In many places, in the event of a dispute over rights to a name (or,
more accurately, DNS label string), this document assumes "first-
come, first-served" (FCFS) as a resolution policy even though FCFS is
not listed below as one of the principles for this document. If
policies are already in place governing priorities and "rights", one
can use the guidelines here by replacing uses of FCFS in this
document with policies specific to the zone. Some of the guidelines
here may not be applicable to other policies for determining rights
to labels. Still other alternatives, such as use of UDRP [UDRP] or
mutual exclusion, might have little impact on other aspects of these
guidelines.
(a) Although some Unicode strings may be pure identifiers made up of
an assortment of characters from many languages and scripts, IDLs are
likely to be "words" or "names" or "phrases" that have specific
meaning in a language. While a zone administration might or might
not require "meaning" as a registration criterion, meaning could
prove to be a useful tool for avoiding user confusion.
Each IDL to be registered should be associated administratively
with one or more languages.
Language associations should either be predetermined by the zone
administrator and applied to the entire zone or be chosen by the
registrants on a per-IDL basis. The latter may be necessary for some
zones, but it will make administration more difficult and will
increase the likelihood of conflicts in variant forms.
A given zone might have multiple languages associated with it or it
may have no language specified at all. Omitting specification of a
language may provide additional opportunities for user confusion and
is therefore NOT recommended.
(b) Each language uses only a subset of Unicode characters.
Therefore, if an IDL is associated with a language, it is not
permitted to contain any Unicode character that is not within the
valid subset for that language.
Each IDL to be registered must be verified against the valid
subset of Unicode for the language(s) associated with the IDL.
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That subset is specified by the list of characters appearing in
the first column of the language and zone-specific tables as
described later in this document.
If the IDL fails this test for any of its associated languages, the
IDL is not valid for registration.
Note that this verification is not necessarily linguistically
accurate, because some languages have special rules. For example,
some languages impose restrictions on the order in which particular
combinations of characters may appear. Characters that are valid for
the language, and hence permitted by this specification, might still
not form valid words or even strings in the language.
(c) When an IDL is associated with a language, it may have Character
Variants that depend on that language associated with it in addition
to any Preferred Variants. These variants are potential sources of
confusion with the Code Points in the original label string.
Consequently, the labels generated from them should be unavailable to
registrants of other names, words, or phrases.
During registration, all labels generated from the Character
Variants for the associated language(s) of the IDL should be
reserved.
IDL reservations of the type described here normally do not appear in
the distributed DNS zone file. In other words, these reserved IDLs
may not resolve. Domain name holders could request that these
reserved IDLs be placed in the zone file and made active and
resolvable.
Zones will need to establish local policies about how they are to be
made active. Specifically, many zones, especially at the top level,
have prohibited or restricted the use of "CNAME"s DNS aliases,
especially CNAMEs that point to nameserver delegation records (NS
records). And long-term use of long-term aliases for domain
hierarchies, rather than single names ("DNAME records") are
considered problematic because of the recursion they can introduce
into DNS lookups.
(d) When an IDL is a "name", "word", or "phrase", it will have
Character Variants depending on the associated language.
Furthermore, one or more of those Character Variants will be used
more often than others for linguistic, political, or other reasons.
These more commonly used variants are distinguished from ordinary
Character Variants and are known as Preferred Variant(s) for the
particular language.
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To increase the likelihood of correct and predictable resolution
of the IDN by end users, all labels generated from the Preferred
Variants for the associated language(s) should be resolvable.
In other words, the Preferred Variant Labels should appear in the
distributed DNS zone file.
(e) IDLs associated with one or more languages may have a large
number of Character Variant Labels or Preferred Variant Labels. Some
of these labels may include combinations of characters that are
meaningless or invalid linguistically. It may therefore be
appropriate for a zone to adopt procedures that include only
linguistically acceptable labels in the IDL Package.
A zone administrator may impose additional rules and other
processing activities to limit the number of Character Variant
Labels or Preferred Variant Labels that are actually reserved or
registered.
These additional rules and other processing activities are based on
policies and/or procedures imposed on a per-zone basis and therefore
are not within the scope of this document. Such policies or
procedures might be used, for example, to restrict the number of
Preferred Variant Labels actually reserved or to prevent certain
words from being registered at all.
(f) There are some Character Variant Labels and Preferred Variant
Labels that are associated with each IDL. These labels are
considered "equivalent" to each another. To avoid confusion, they
all should be assigned to a single domain name holder.
The IDL and its variant labels should be grouped together into a
single atomic unit, known in this document as an "IDL Package".
The IDL Package is created upon registration and is atomic: Transfer
and deletion of an IDL is performed on the IDL Package as a whole.
That is, an IDL within the IDL Package may not be transferred or
deleted individually; any re-registration, transfers, or other
actions that impact the IDL should also affect the other variants.
The name-conflict resolution policy associated with this zone could
result in a conflict with the principle of IDL Package atomicity. In
such a case, the policy must be defined to make the precedence clear.
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RFC 3743 JET Guidelines for IDN April 20043.2. Registration of IDL
To conform to the principles described in 3.1, this document
introduces two concepts: the Language Variant Table and the IDL
Package. These are described in the next two subsections, followed
by a description of the algorithm that is used to interpret the table
and generate variant labels.
3.2.1. Using the Language Variant Table
For each zone that uses a given language, each language should have
its own Language Variant Table. The table consists of a header
section that identifies references and version information, followed
by a section with one row for each Code Point that is valid for the
language and three columns.
(1) The first column contains the subset of Unicode characters
that is valid to be registered ("Valid Code Point"). This is
used to verify the IDL to be registered (see 3.1b). As in the
registration procedure described later, this column is used as
an index to examine characters that appear in a proposed IDL
to be processed. The collection of Valid Code Points in the
table for a particular language can be thought of as defining
the script for that language, although the normal definition
of a script would not include, for example, ASCII characters
with CJK ones.
(2) The second column contains the Preferred Variant(s) of the
corresponding Unicode character in column one ("Valid Code
Point"). These variant characters are used to generate the
Preferred Variant Labels for the IDL. Those labels should be
resolvable (see 3.1d). Under normal circumstances, all of
those Preferred Variant Labels will be activated in the
relevant zone file so that they will resolve when the DNS is
queried for them.
(3) The third column contains the Character Variant(s) for the
corresponding Valid Code Point. These are used to generate
the Character Variant Labels of the IDL, which are then to be
reserved (see 3.1c). Registration, or activation, of labels
generated from Character Variants will normally be a
registrant decision, subject to local policy.
Each entry in a column consists of one or more Code Points, expressed
as a numeric character number in the Unicode table and optionally
followed by a parenthetical reference. The first column, or Valid
Code Point, may have only one Code Point specified in a given row.
The other columns may have more than one.
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Any row may be terminated with an optional comment, starting with
"#".
The formal syntax of the table and more-precise definitions of some
of its organization appear in Section 5.
The Language Variant Table should be provided by a relevant group,
organization, or body. However, the question of who is relevant or
has the authority to create this table and the rules that define it
is beyond the scope of this document.
3.2.2. IDL Package
The IDL Package is created on successful registration and consists
of:
(1) the IDL registered
(2) the language(s) associated with the IDL
(3) the version of the associated character variant table
(4) the reserved IDLs
(5) active IDLs, that is, "Zone Variant Labels" that are to appear
in the DNS zone file
3.2.3. Procedure for Registering IDLs
An explanation follows each step.
Step 1. IN <= IDL to be registered and
{L} <= Set of languages associated with IN
Start the process with the label string (prospective IDL) to be
registered and the associated language(s) as input.
Step 2. Generate the Nameprep-processed version of the IN,
applying all mappings and canonicalization required by
IDNA.
The prospective IDL is processed by using Nameprep to apply the
normalizations and exclusions globally required to use IDNA. If the
Nameprep processing fails, then the IDL is invalid and the
registration process must stop.
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Step 2.1. NP(IN) <= Nameprep processed IN
Step 2.2. Check availability of NP(IN). If not available, route to
conflict policy.
The Nameprep-processed IDL is then checked against the contents of
the zone file and previously created IDL Packages. If it is already
registered or reserved, then a conflict exists that must be resolved
by applying whatever policy is applicable for the zone. For example,
if FCFS is used, the registration process terminates unless the
conflict resolution policy provides another alternative.
Step 3. Process each language.
For each language (AL) in {L}
Step 3 goes through all languages associated with the proposed IDL
and checks each character (after Nameprep has been applied) for
validity in each of them. It then applies the Preferred Variants
(column 2 values) and the Character Variants (column 3 values) to
generate candidate labels.
Step 3.1. Check validity of NP(IN) in AL. If failed, stop
processing.
In step 3.1, IDL validation is done by checking that every Code Point
in the Nameprep-processed IDL is a Code Point allowed by the "Valid
Code Point" column of the Character Variant Table for the language.
This is then repeated for any other languages (and hence, Language
Variant Tables) specified in the registration. If one or more Code
Points are not valid, the registration process terminates.
Step 3.2. PV(IN,AL) <= Set of available Nameprep-processed Preferred
Variants of NP(IN) in AL
Step 3.2 generates the list of Preferred Variant Labels of the IDL by
doing a combination (see Step 3.2A below) of all possible variants
listed in the "Preferred Variant(s)" column for each Code Point in
the Nameprep-processed IDL. The generated Preferred Variant Labels
must be processed through Nameprep. If the Nameprep processing fails
for any Preferred Variant Label (this is unlikely to occur if the
Preferred Variants are processed through Nameprep before being placed
in the table), then that variant label will be removed from the list.
The remaining Preferred Variant Labels in the list are then checked
to see whether they are already registered or reserved. If any are
registered or reserved, then the conflict resolution policy will
apply. In general, this will not prevent the originally requested
IDL from being registered unless the policy prevents such
registration. For example, if FCFS is applied, then the conflicting
variants will be removed from the list, but the originally requested
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RFC 3743 JET Guidelines for IDN April 2004
IDL and any remaining variants will be registered (see steps 5 and 8
below).
Step 3.2A Generating variant labels from Variant Code Points.
Steps 3.2 and 3.3 require that the Preferred Variants and Character
Variants be combined with the original IDL to form sets of variant
labels. Conceptually, one starts with the original, Nameprep-
processed, IDL and examines each of its characters in turn. If a
character is encountered for which there is a corresponding Preferred
Variant or Character Variant, a new variant label is produced with
the Variant Code Point substituted for the original one. If variant
labels already exist as the result of the processing of characters
that appeared earlier in the original IDL, then the substitutions are
made in them as well, resulting in additional generated variant
labels. This operation is repeated separately for the Preferred
Variants (in Step 3.2) and Character Variants (in Step 3.3). Of
course, equivalent results could be achieved by processing the
original IDL's characters in order, building the Preferred Variant
Label set and Character Variant Label set in parallel.
This process will sometimes generate a very large number of labels.
For example, if only two of the characters in the original IDL are
associated with Preferred Variants and if the first of those
characters has three Preferred Variants and the second has two, one
ends up with 12 variant labels to be placed in the IDL Package and,
normally, in the zone file. Repeating the process for Character
Variants, if any exist, would further increase the number of labels.
And if more than one language is specified for the original IDL, then
repetition of the process for additional languages (see step 4,
below) might further increase the size of the set.
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RFC 3743 JET Guidelines for IDN April 2004
For illustrative purposes, the "combination" process could be
achieved by a recursive function similar to the following pseudocode:
Function Combination(Str)
F <= first codepoint of Str
SStr <= Substring of Str, without the first code point
NSC <= {}
If SStr is empty then
for each V in (Variants of code point F)
NSC = NSC set-union (the string with the code point V)
End of Loop
Else
SubCom = Combination(SStr)
For each V in (Variants of code point F)
For each SC in SubCom
NSC = NSC set-union (the string with the
first code point V followed by the string SC)
End of Loop
End of Loop
Endif
Return NSC
Step 3.3. CV(IN,AL) <= Set of available Nameprep-processed Character
Variants of NP(IN) in AL
This step generates the list of Character Variant Labels by doing a
combination (see Step 3.2A above) of all the possible variants listed
in the "Character Variant(s)" column for each Code Point in the
Nameprep-processed original IDL. As with the Preferred Variant
Labels, the generated Character Variant Labels must be processed by,
and acceptable to, Nameprep. If the Nameprep processing fails for a
Character Variant Label, then that variant label will be removed from
the list. The remaining Character Variant Labels are then checked to
be sure they are not registered or reserved. If one or more are,
then the conflict resolution policy is applied. As with Preferred
Variant Labels, a conflict that is resolved in favor of the earlier
registrant does not, in general, prevent the IDL from being
registered, nor the remaining variants from being reserved in step 6
below.
Step 3.4. End of Loop
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RFC 3743 JET Guidelines for IDN April 2004
Step 4. Let PVall be the set-union of all PV(IN,AL)
Step 4 generates the Preferred Variants Label for all languages. In
this step, and again in step 6 below, the zone administrator may
impose additional rules and processing activities to restrict the
number of Preferred (tentatively to be reserved and activated) and
Character (tentatively to be reserved) Label Variants. These
additional rules and processing activities are zone policy specific
and therefore are not specified in this document.
Step 5. {ZV} <= PVall set-union NP(IN)
Step 5 generates the initial Zone Variants. The set includes all
Preferred Variants for all languages and the original Nameprep-
processed IDL. Unless excluded by further processing, these Zone
Variants will be activated. That is, placed into the DNS zone. Note
that the "set-union" operation will eliminate any duplicates.
Step 6. Let CVall be the set-union of all CV(IN,AL), set-minus
{ZV}
Step 6 generates the Reserved Label Variants (the Character Variant
Label set). These labels are normally reserved but not activated.
The set includes all Character Variant Labels for all languages, but
not the Zone Variants defined in the previous step. The set-union
and set-minus operations eliminate any duplicates.
Step 7. Create IDL Package for IN using IN, {L}, {ZV} and CVall
In Step 7, the "IDL Package" is created using the original IDL, the
associated language(s), the Zone Variant Labels, and the Reserved
Variant Labels. If zone-specific additional processing or filtering
is to be applied to eliminate linguistically inappropriate or other
forms, it should be applied before the IDL Package is actually
assembled.
Step 8. Put {ZV} into zone file
The activated IDLs are converted via ToASCII with UseSTD13ASCIIRules
[IDNA] before being placed into the zone file. This conversion
results in the IDLs being in the actual IDNA ("Punycode") form used
in zone files, while the IDLs have been carried in Unicode form up to
this point. If ToASCII fails for any of the activated IDLs, that IDL
must not be placed into the zone file. If the IDL is a subdomain
name, it will be delegated.
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RFC 3743 JET Guidelines for IDN April 20043.3. Deletion and Transfer of IDL and IDL Package
In traditional domain administration, every Domain Name Label is
independent of all other Domain Name Labels. Registration, deletion,
and transfer of labels is done on a per-label basis. However, with
the guidelines discussed here, each IDL is associated with specific
languages, with all label variants, both active (zone) and reserved,
together in an IDL Package. This quite deliberately prohibits labels
that contain sufficient mixtures of characters from different scripts
to make them impossible as words in any given language. If a zone
chooses to not impose that restriction--that is, to permit labels to
be constructed by picking characters from several different languages
and scripts--then the guidelines described here would be
inappropriate.
As stated earlier, the IDL package should be treated as a single
atomic unit and all variants of the IDL should belong to a single
domain-name holder. If the local policy related to the handling of
disagreements requires a particular IDL to be transferred and deleted
independently of the IDL Package, the conflict policy would take
precedence. In such an event, the conflict policy should include a
transfer or delete procedure that takes the nature of IDL Packages
into consideration.
When an IDL Package is deleted, all of the Zone and Reserved Label
Variants again become available. The deletion of one IDL Package
does not change any other IDL Packages.
3.4. Activation and Deactivation of IDL variants
Because there are active (registered) IDLs and inactive (reserved but
not registered) IDLs within an IDL package, processes are required to
activate or deactivate IDL variants within an IDL Package.
3.4.1. Activation Algorithm
Step 1. IN <= IDL to be activated and PA <= IDL Package
Start with the IDL to be activated and the IDL Package of which it is
a member.
Step 2. NP(IN) <= Nameprep processed IN
Process the IDL through Nameprep. This step should never cause a
problem, or even a change, since all labels that become part of the
IDL Package are processed through Nameprep in Step 3.2 or 3.3 of the
Registration procedure (section 3.2.3).
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RFC 3743 JET Guidelines for IDN April 2004
Step 3. If NP(IN) not in CVall then stop
Verify that the Nameprep-processed version of the IDL appears as a
still-unactivated label in the IDL Package, i.e., in the list of
Reserved Label Variants, CVall. It might be a useful "sanity check"
to also verify that it does not already appear in the zone file.
Step 4. CVall <= CVall set-minus NP(IN) and {ZV} <= {ZV} set-union
NP(IN)
Within the IDL Package, remove the Nameprep-processed version of the
IDL from the list of Reserved Label Variants and add it to the list
of active (zone) label variants.
Step 5. Put {ZV} into the zone file
Actually register (activate) the Zone Variant Labels.
3.4.2. Deactivation Algorithm
Step 1. IN <= IDL to be deactivated and PA <= IDL Package
As with activation, start with the IDL to be deactivated and the IDL
Package of which it is a member.
Step 2. NP(IN) <= Nameprep processed IN
Get the Nameprep-processed version of the name (see discussion in the
previous section).
Step 3. If NP(IN) not in {ZV} then stop
Verify that the Nameprep-processed version of the IDL appears as an
activated (zone) label variant in the IDL Package. It might be a
useful "sanity check" at this point to also verify that it actually
appears in the zone file.
Step 4. CVall <= CVall set-union NP(IN) and {ZV} <= {ZV} set-minus
NP(IN)
Within the IDL Package, remove the Nameprep-processed version of the
IDL from the list of Active (Zone) Label Variants and add it to the
list of Reserved (but inactive) Label Variants.
Step 5. Put {ZV} into the zone file
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RFC 3743 JET Guidelines for IDN April 20043.5. Managing Changes in Language Associations
Since the IDL package is an atomic unit and the associated list of
variants must not be changed after creation, this document does not
include a mechanism for adding and deleting language associations
within the IDL package. Instead, it recommends deleting the IDL
package entirely, followed by a registration with the new set of
languages. Zone administrators may find it desirable to devise
procedures that prevent other parties from capturing the labels in
the IDL Package during these operations.
3.6. Managing Changes to the Language Variant Tables
Language Variant Tables are subject to changes over time, and these
changes may or may not be backward compatible. It is possible that
updated Language Variant Tables may produce a different set of
Preferred Variants and Reserved Variants.
In order to preserve the atomicity of the IDL Package, when the
Language Variant Table is changed, IDL Packages created using the
previous version of the Language Variant Table must not be updated or
affected.
4. Examples of Guideline Use in Zones
To provide a meaningful example, some Language Variant Tables must be
defined. Assume, then, for the purpose of giving examples, that the
following four Language Variant Tables are defined:
Note: these tables are not a representation of the actual tables, and
they do not contain sufficient entries to be used in any actual
implementation. IANA maintains a voluntary registry of actual tables
[IANA-LVTABLES] which may be consulted for complete examples.
a) Language Variant Table for zh-cn and zh-sg
Reference 1 CP936 (commonly known as GBK)
Reference 2 zVariant, zTradVariant, zSimpVariant in Unihan.txt [UNIHAN]
Reference 3 List of Simplified character Table (Simplified column)
Reference 4 zSimpVariant in Unihan.txt [UNIHAN]
Reference 5 variant that exists in GB2312, common simplified hanzi
Version 1 20020701 # July 2002
56E2(1);56E2(5);5718(2) # sphere, ball, circle; mass, lump
5718(1);56E2(4);56E2(2),56E3(2) # sphere, ball, circle; mass, lump
60F3(1);60F3(5); # think, speculate, plan, consider
654E(1);6559(5);6559(2) # teach
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RFC 3743 JET Guidelines for IDN April 2004
Every language should have its own Language Variant Table provided by
a relevant group, organization, or other body. That table will
normally be based on some established standard or standards. The
group that defines a Language Variant Table should document
references to the appropriate standards at the beginning of the
table, tagged with the word "Reference" followed by an integer (the
reference number) followed by the description of the reference. For
example:
Reference 1 CP936 (commonly known as GBK)
Reference 2 zVariant, zTradVariant, zSimpVariant in Unihan.txt
Reference 3 List of Simplified Character Table (Simplified column)
Reference 4 zSimpVariant in Unihan.txt
Reference 5 Variant that exists in GB2312, common simplified Hanzi
Each Language Variant Table must have a version number and its
release date. This is tagged with the word "Version" followed by an
integer then followed by the date in the format YYYYMMDD, where YYYY
is the 4-digit year, MM is the 2-digit month, and DD is the 2-digit
day of the publication date of the table.
Version 1 20020701 # July 2002 Version 1
The table has three columns, separated by semicolons: "Valid Code
Point"; "Preferred Variant(s)"; and "Character Variant(s)".
The "Valid Code Point" is the subset of Unicode characters that are
valid to be registered.
There can be more than one Preferred Variant; hence there could be
multiple entries in the "Preferred Variant(s)" column. If the
"Preferred Variant(s)" column is empty, then there is no
corresponding Preferred Variant; in other words, the Preferred
Variant is null, there is no corresponding preferred variant
codepoint, and no processing to add labels for preferred variants
occurs." Unless local policy dictates otherwise, the procedures
above will result in only those labels that reflect the valid code
point being activated (registered) into the zone file.
The "Character Variant(s)" column contains all Character Variants of
the Code Point. Since the Code Point is always a variant of itself,
to avoid redundancy, the Code Point is assumed to be part of the
"Character Variant(s)" and need not be repeated in the "Character
Variant(s)" column.
If the variant in the "Preferred Variant(s)" or the "Character
Variant(s)" column is composed of a sequence of Code Points, then
sequence of Code Points is listed separated by a space.
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RFC 3743 JET Guidelines for IDN April 2004
If there are multiple variants in the "Preferred Variant(s)" or the
"Character Variant(s)" column, then each variant is separated by a
comma.
Any Code Point listed in the "Preferred Variant(s)" column must be
allowed by the rules for the relevant language to be registered.
However, this is not a requirement for the entries in the "Character
Variant(s)" column; it is possible that some of those entries may not
be allowed to be registered.
Every Code Point in the table should have a corresponding reference
number (associated with the references) specified to justify the
entry. The reference number is placed in parentheses after the Code
Point. If there is more than one reference, then the numbers are
placed within a single set of parentheses and separated by commas.
6. Security Considerations
As discussed in the Introduction, substantially-unrestricted use of
international (non-ASCII) characters in domain name labels may cause
user confusion and invite various types of attacks. In particular,
in the case of CJK languages, an attacker has an opportunity to
divert or confuse users as a result of different characters (or, more
specifically, assigned code points) with identical or similar
semantics. These Guidelines provide a partial remedy for those risks
by supplying a framework for prohibiting inappropriate characters
from being registered at all and for permitting "variant" characters
to be grouped together and reserved, so that they can only be
registered in the DNS by the same owner. However, the system it
suggests is no better or worse than the per-zone and per-language
tables whose format and use this document specifies. Specific
tables, and any additional local processing, will reflect per-zone
decisions about the balance between risk and flexibility of
registrations. And, of course, errors in construction of those
tables may significantly reduce the quality of protection provided.
7. Index to Terminology
As a convenience to the reader, this section lists all of the special
terminology used in this document, with a pointer to the section in
which it is defined.
Activated Label 2.1.17
Activation 2.1.4
Active Label 2.1.17
Character Variant 2.1.14
Character Variant Label 2.1.16
CJK Characters 2.1.9
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RFC 3743 JET Guidelines for IDN April 2004
[ISO7098] ISO 7098;1991 Information and documentation
Romanization of Chinese, ISO/TC46/SC2.
9.2. Informative References
[IANA-LVTABLES] Internet Assigned Numbers Authority (IANA), IDN
Character Registry.
http://www.iana.org/assignments/idn/
[IDN-WG] IETF Internationalized Domain Names Working Group,
now concluded,idn@ops.ietf.org, James Seng, Marc
Blanchet, co-chairs, http://www.i-d-n.net/.
[UDRP] ICANN, "Uniform Domain Name Dispute Resolution
Policy", October 1999,
http://www.icann.org/udrp/udrp-policy-24oct99.htm
[ISO639] "ISO 639:1988 (E/F) Code for the representation of names
of languages", International Organization for
Standardization, 1st edition, 1988-04-01.
10. Contributors
The formal responsibility for this document and the ideas it contains
lie with K. Koniski, K. Huang, H. Qian, and Y. Ko. These authors are
listed on the first page as authors of record, and they are the
appropriate the long-term contacts for questions and comments on this
RFC. On the other hand, J. Seng, J. Klensin, and W. Rickard served
as editors of the document, transcribing and translating the ideas of
the four authors and the teams they represented into the current
written form. They were the primary contacts during the editing
process, but not in the long term.
Konishi, et al. Informational [Page 30]

RFC 3743 JET Guidelines for IDN April 200411. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
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made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
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Copies of IPR disclosures made to the IETF Secretariat and any
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The IETF invites any interested party to bring to its attention any
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this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Konishi, et al. Informational [Page 33]